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2.2.1Space

Light-Years — SQA National 5 Physics Revision Notes & Topical Questions

5 min read 5 parts Lesson 8 of 12

📚 SQA National 5 Physics Hub / Topical Revision
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Part 1 of 5What Is a Light-Year?

2.2.1 Light-Years

Distances between stars are so large that metres and kilometres become awkward.

Definition
  • A light-year is the distance travelled by light in a vacuum in one year, so it is a unit of distance.
Not a time
  • Despite the word year, a light-year measures length, just like a kilometre or a mile.
Size
  • One light-year is about 9.46 × 10¹⁵ m, which is nearly ten million million kilometres.
Typical values
  • The Milky Way is roughly 100 000 light-years across, and the nearest star beyond the Sun is over four light-years away.

Part 2 of 5Calculating the Distance

Light travels at a constant speed, so the equation d = vt applies directly.

Calculating the Distance
Speed of light
  • In a vacuum light travels at 3.00 × 10⁸ m/s, which is the fastest speed possible.
Seconds in a year
  • One year is 60 × 60 × 24 × 365 = 31 536 000 s, or about 3.15 × 10⁷ s.
Using d = vt
  • Multiplying 3.00 × 10⁸ m/s by 3.15 × 10⁷ s gives roughly 9.46 × 10¹⁵ m per light-year.
Converting
  • Multiply a distance in light-years by 9.46 × 10¹⁵ to obtain metres, and divide to go the other way.
Time for light to travelApproximate distance
1 second3.00 × 10⁸ m
500 seconds (Sun to Earth)1.5 × 10¹¹ m
1 year9.46 × 10¹⁵ m

Part 3 of 5Looking Back in Time

Light takes time to cross space, so what we see is always out of date.

Delay
  • Light from a star 8 light-years away left it 8 years ago, so we see the star as it was then.
Nearby objects
  • Sunlight takes about 500 s to reach Earth and moonlight about 1.3 s, so delays are small locally.
Distant galaxies
  • Light from far galaxies set off millions or billions of years ago, giving a picture of the early universe.
Why it matters
  • Astronomers can study how the universe has changed simply by observing objects at increasing distances.

Part 4 of 5Worked Example

Question

  • A star is 8.0 light-years from Earth.
  • Calculate this distance in metres, taking one light-year as 9.46 × 10¹⁵ m.

  1. Distance in light-years = 8.0.
  2. d = 8.0 × 9.46 × 10¹⁵ = 7.568 × 10¹⁶ m.
  3. Round to two significant figures to match the data.

Answer The star is about 7.6 × 10¹⁶ m away.

Part 5 of 5Exam Checklist & Tips

2.2.1 – Key Points for the Exam

  • Light-year The distance light travels in a vacuum in one year.
  • Speed of light 3.00 × 10⁸ m/s in a vacuum.
  • d = vt Distance equals speed multiplied by time.
  • Astronomical distance A very large distance between stars or galaxies.

Common Mistakes

  • Thinking a light-year is a unit of time because it contains the word year.
  • Using minutes or hours instead of seconds in d = vt, which gives the wrong answer.
  • Forgetting to give the answer in standard form with the correct power of ten.
  • Using the speed of sound instead of the speed of light.

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